Put brats in a large pot, add onions, beer and enough water to cover brats. Bring to a simmer over medium heat. Reduce heat to medium-low and simmer gently 15 to 20 minutes. Don’t let the liquid boil, or the brats might burst.
Using tongs, remove brats from water. Oil the grill grate. Place brats on grate and cook, turning every few minutes with tongs, 10 to 15 minutes or until nicely browned and sizzling. Put brats in rolls and serve immediately with mustard. Serves 5 to 10.